1969 Charger--Convert Ammeter to Voltmeter

[Chatt69chgr]

Has anyone hired Redline Gauge Works to make this conversion?  What did Redline do and what did they charge?  I'm asking here since I can't get Redline to answer the phone. 
There would appear to be two ways to do this.  One is to buy a small voltmeter (like a Bosch 8215) and do the appropriate mechanical engineering to fit it into the existing rallye dash hole.  They second would be to just use the existing ammeter.  Disconnect the wiring to it and install a resistor in series with the ammeter. Glue a new scale onto the existing ammeter faceplate.  I'm not sure what size resistor would be required. Maybe use a variable resistor like a 500K or 1 Meg Ohm potentiometer in series tied to a 13.6V source and vary the resistance until the gauge needle is at the midpoint.  Remove potentiometer and insert fixed resistor.  Then vary the voltage source and note the gauge needle positions for say even voltage increments from 8-14 volts?  Would need to make a paper gauge face to glue to the existing gauge. Just a thought.
So has anyone chased this rabbit? 

[b5blue]

I think Dino did that? 

[John_Kunkel]

https://www.youtube.com/watch?v=JtXOS_jOHfQ